The present invention relates to current mirrors and, more particularly, to an integrated current mirror for providing a large scale up or scale down factor within a small geometry.
Integrated current mirror circuits are well known in the art. The most common type of integrable current mirror comprises a diode of which the anode is coupled to the base of an output transistor, while the cathode and emitter of the two devices are coupled to ground potential. Typically, the diode is fabricated by shorting the collector and base electrode of a transistor together to the base of the transistor (the anode) while the emitter of the diode connected transistor is returned to ground potential. The collector of the output transistor sinks a current the magnitude of which may be scaled with respect to the magnitude of input current source to the anode of the diode.
If it is desired to scale up the output current flowing through the collector of the output transistor, ie, to have a larger output current than supplied input current, it is typical that the emitter of the output transistor is made N times larger than the emitter of the diode connected transistor. Thus, the output current will be N times the magnitude of the applied input current. Conversely, by making the emitter of the diode connected transistor N times larger than the emitter area of the output transistor, the output current will be N times less than the magnitude of the input current. However, if a large scaling factor, i.e, N greater than 10, is desired, the above described approach is undesirable as a large collector-substrate capacitance results in the larger emitter device. Moreover, the current density of the larger device is reduced which is undesirable in high speed, low current applications.
Another method for scaling the output current with respect to the input current is to place a resistor in series with one of the other of the two transistors. For instances, if the output current is to be scaled down with respect to the input current, a resistor can be added in series with the emitter of the output transistor. The problem with this approach is that although an accurate scaling is achieved at one specified current level, the scaling factor changes with temperature in the applied magnitude of input current which causes undesirable nonlinearities over temperature and is not a good choice for alternating current applications.
Hence, a need exists for an improved current mirror that is suited to be manufactured in integrated circuit form and which provides a large current scaling factor with small geometry.